GUISE, GUISE,** Calm the **** down.**
Episode 42 - "The Big Picture" has a scene in which they flip through the annual picture book. It shows Frankie growing up from a little girl to her current age. And since imaginary friends never age, the theory is disproved.
Now calm your tits, everyone.

Also, Madame Foster imagined the rabbit thing (I don't remember his name, sir harrington or something such as that) so it would not be possibru for Madame Foster to imagine Frankie since people can only imagine 1 friend.

what if they dont age because they werent imagined to age, so when miss foster imagined her she wanted it to experience a full life and not that of just an "imaginary friend" but a whole new life with potential?